Technical Field
The present disclosure relates to a position detecting device used with a capacitive stylus pen and a position detecting method.
Description of the Related Art
In recent years, as disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2014-063249) for example, a position detecting device used with a capacitive stylus pen has been provided. The position detecting device disclosed in this Patent Document 1 includes a transparent sensor and this transparent sensor is disposed so as to overlap with a display screen of a display device so that input of handwritten characters and drawing of pictures, illustrations, etc., which are difficult with a finger, can be easily performed through input with a stylus pen.
In this Patent Document 1, the capacitive stylus pen transmits an alternating current (AC) signal to electrodes provided in the sensor of the position detecting device by electric field coupling. Meanwhile, in the position detecting device, the position of the electrode that has received the AC signal from the stylus pen by the electric field coupling is detected and thereby the position indicated on the sensor by the stylus pen is detected.
Furthermore, in the position detecting device disclosed in this Patent Document 1, a multiplexer that selects two electrodes among the plural electrodes provided in the sensor and signals from the two electrodes that are selected by the multiplexer and are separated from each other by a certain distance are amplified by a differential amplifier to eliminate the influence of external noise.
Moreover, in Patent Document 2 (Japanese Patent Laid-Open No. Hei 8-95701), a transparent digitizer serving also as a touch panel is disclosed. This digitizer is configured so as to cancel out noise of a common mode by amplifying signals from two electrodes adjacent to each other by a differential amplifier circuit.
The external noise has the same influence on the two electrodes adjacent to each other. Therefore, if the signals from the adjacent two electrodes are subjected to differential amplification by the differential amplifier circuit as in the digitizer of Patent Document 2, the influence of the external noise can be favorably cancelled.
However, when the stylus pen is separate from the sensor surface by a predetermined distance in the height direction, i.e., is in a so-called hover state, the degrees of electric field coupling with the stylus pen are similar to each other between two electrodes that are located closest to this stylus pen in the hover state and are adjacent to each other. Therefore, when differential amplification is performed on the respective signals generated in these adjacent two electrodes, not only the external noise but also a signal from the stylus pen is cancelled. For this reason, in the case of selecting electrodes adjacent to each other as in the digitizer of Patent Document 2, it might be impossible to favorably detect the stylus pen in the hover state.
In contrast to this, in the position detecting device of the above-described Patent Document 1, two electrodes connected to the positive-side input terminal (non-inverting input terminal) and the negative-side input terminal (inverting input terminal) of a differential amplifier are so selected as to be always separate from each other by a constant distance on the sensor. Thus, a difference is generated in the electric field coupling with the stylus pen in the hover state between the two electrodes as the target of differential amplification processing. Therefore, in the position detecting device of Patent Document 1, external noise can be cancelled by the differential amplification processing and a signal from the stylus pen can be detected. However, because the distance between the selected two electrodes is always set constant, possibly the stylus pen cannot necessarily be favorably detected depending on the degree of the hover state of the stylus pen, i.e., the height of the stylus pen from the sensor.
In addition, in operation of the position detecting device, in some cases, a position is indicated on the sensor with a stylus pen held by a hand in a state in which a finger or hand is in contact with the sensor of the position detecting device. When a hand or finger is in contact with the sensor of the position detecting device as above, external noise often enters the respective electrodes connected to the input terminals of the differential amplifier from the hand or finger.
If the respective electrodes connected to the positive-side input terminal and the negative-side input terminal of the differential amplifier are electrodes that are so selected as to be always separated from each other by a fixed distance as in the position detecting device of Patent Document 1, generally the influence of the entry of external noise due to the contact of a hand or finger is not evenly exerted on both the positive-side input terminal and the negative-side input terminal of the differential amplifier. Therefore, in the position detecting device of Patent Document 1, the influence of the entry of external noise due to the contact of a hand or finger with the sensor cannot be eliminated by the differential amplification processing in some cases. For this reason, a noise signal is superimposed on a signal from the stylus pen and the signal of the stylus pen cannot be successfully detected. Thus, possibly the detection accuracy of the position indicated by the stylus pen significantly deteriorates.